When weight control is required, bottle-filling machines are currently equipped with dynamometric scale systems, which at most provide for sample checking a percentage of production, on account of the intrinsic slowness of mechanical weighing systems with respect to the operating speed of the machine.
In the pharmaceutical industry, however, there is increasing demand for 100% control of the content of bottles, particularly of certain critical and/or high-cost (e.g., antitumor) drugs requiring highly precise dosage control.
The main purpose of 100% control is to ensure all the bottles are filled correctly, thus ensuring safe use of the product. 100% control also provides for obtaining, much faster than sample weighing methods, significant information concerning statistical production parameters, such as mean value and spread, which are vital for controlling correct adjustment and operation, respectively, of the metering device.
The importance of 100% control of liquids is also shown by the large sums invested in developing bulky, high-cost, intricate nuclear magnetic resonance systems for installation downstream from the filling section.
Capacitive measuring techniques are known and widely used, particularly for measuring movement, but to a much lesser degree for measuring material mass, though significant examples are known in the tobacco industry, and particularly in the pharmaceutical industry, relative to solid dielectric materials.
As regards liquids, the capacitive technique is frequently used for level measurement, one possible known embodiment of which is shown in attached FIG. 1.
In this, the variation in capacitance between two electrodes EL1, EL2—immersed in a liquid LQ or fitted, adjacent to the liquid, to the wall PT of a container CT—is proportional to level LV by virtue of the relative dielectric constant of liquid LQ.
As stated, the capacitive technique is also known to be used for measuring the solid content (powder, granules, tablets or microtablets) of pharmaceutical capsules using the gross-tare technique, as described, for example, in the Applicant's U.S. Pat. No. 5,750,938.